Aqueous dispersions containing polymers such as paints, stains, adhesives, and printing inks are being increasingly used because of their safety, economy, ease of application, and ease of clean-up. Aqueous coatings such as latex paints, dry quickly compared to solvent coatings such as alkyd paints. Because of the faster drying time of aqueous coatings relative to solvent coatings, a second coat can be applied in a short time, for example, from about 3 to about 5 hours. However, because of the fast drying nature of aqueous coatings, there is often not enough time to rebrush over the freshly coated wet surface to improve its appearance or to apply additional paint onto the freshly coated wet surface without causing defects such as brush marks, loss of gloss, or lap lines in the final dried coating. A "lap" as used herein, refers to an area on a substrate where additional coating is applied onto a portion of a previously coated, but still wet, adjacent substrate area.
In the case of paint, it is desirable to seamlessly join up the edges of a wet painted area with newly added fresh paint without any visible lap showing in the dried coating. "Lapping" or "to lap" refers to the ability to do this seamless joining of edges without leaving a visible "lap" line.
As used herein, "open time" or "wet-edge time" refers to the time that a coating remains workable, after it has been applied to a substrate, to allow for rebrushing or "melting in" of the newly applied coating at the lap, without resulting in the above defects. Open time is a more apparent problem in cases where other than a flat or dull surface appearance is desired, such as with sheen and gloss paints, because a glossy appearance makes the surface defects more visible.
During the drying of the aqueous coating, as water evaporates, latex polymer particles deform and fuse together to form a continuous film. This process, known as "film formation" or "coalescence", is irreversible since the addition of water onto the dried or drying coating will not redisperse the polymer particles. Once coalescence occurs, it is impossible to rebrush the aqueous coating and lap into a previously coated area.
The drying characteristics of aqueous coatings such as dry time, flow and gloss of the dried coating, are not easily controlled, because they are dependent on how fast the water in the coating evaporates. The drying behavior of aqueous coatings depends on the temperature, humidity and air velocity at the time of application and thereafter. For example, coatings can dry almost instantaneously under hot, dry, or windy conditions leading to poor lapping characteristics and short open times. In addition, these conditions tend to also impair the quality of film formation which can result in cracks, poor flow, low gloss, and poor adhesion to the substrate.
Polymeric compositions dispersed in organic solvents have different drying characteristics than the aqueous polymeric coatings because solvents or solvent blends tend to have a wide range of boiling points and evaporation rates compared to water. This range of boiling points can be used to modify the drying characteristics of the solvent based coating according to the specific application requirements. For example, alkyd paints are made with hydrocarbon solvents which are selected so that the drying rate is sufficient to minimize sagging, running, and dust pickup, while permitting good "melting in" at the lap. The disadvantages of such organic solvent based coatings include the difficulties in clean-up, the toxic nature of organic solvents, pollution, odor, stickiness of the dried paint film, such as for example, tack, and the relatively high cost of organic solvents.
The conventional practice for addressing the quick drying nature of aqueous coatings has been to add substantial levels of water-soluble solvents, such as for example, 10 to 20 percent or more based on total liquid content, to such coatings to increase the open time. These techniques are discussed by M. D. Andrews, "Influence of Ethylene and Propylene Glycols on Drying Characteristics of Latex Paints," Journal of Paint Technology, vol. 46, page 40 (1974); D. A. Sullivan, "Water and Solvent Evaporation from Latex and Latex Paint Films," Journal of Paint Technology, vol. 47, page 60 (1975); and C. R. Martens, Waterborne Coatings, Van Nostrand Reinhold, page 153 (1981). These references discuss the use of short chain water-soluble alcohols and glycols as cosolvents in aqueous coatings to aid coalescence of the latex particles, improve leveling, and prolong open time. However, these cosolvents negate the intrinsic advantages of aqueous coatings such as safety, low tack, low odor, and low pollution.